A thing that detects the position of an object through transmission and reception of an ultrasonic signal is basically one that measures the distance on the basis of a time lag between transmission of an ultrasonic signal and reception of a reflected wave from an object. In particular, an object detecting device using a transmission tube for allowing an ultrasonic wave to pass therethrough is illustrated in Patent Document 1.
FIG. 1 is a cross-sectional view of an ultrasonic transmitting and receiving unit of a device for detecting an object on a road illustrated in Patent Document 1. The ultrasonic transmitting and receiving unit includes a cylindrical transmitting tube 10, and an ultrasonic generator 20 is fixed at a first end of the transmitting tube 10, and a second end of the transmitting tube 10 is closed. An ultrasonic wave emitted from the generator 20 propagates through the inside of the transmitting tube 10 and is emitted to the outside through openings 30(1) to 30 (10) and horns 31(1) to 31(10). A reflected wave from an object re-enters the transmitting tube 10, propagates therethrough, and returns to the generator 20.
In such a way, a circuit connected to the generator 20 determines the distance to the object on the basis of a time lag from the transmission of the above ultrasonic signal to the reception.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-96769